Intra-abdominal hypertension (IAH) is an important yet frequently under-recognized complication among critically ill and surgical patients. Persistent elevation of intra-abdominal pressure (IAP) adversely affects cardiovascular, respiratory, renal, gastrointestinal, and neurological function and may progress to abdominal compartment syndrome (ACS), resulting in multiorgan dysfunction and increased mortality if not recognized early [1,2]. According to the World Society of the Abdominal Compartment Syndrome (WSACS), IAH is defined as a sustained or repeated pathological elevation of IAP of ≥12 mmHg, whereas ACS is defined as a sustained IAP >20 mmHg accompanied by new-onset organ dysfunction or failure [2]. Despite growing awareness, IAH remains underdiagnosed in intensive care units owing to inadequate surveillance and the invasive nature of standard monitoring techniques [3]. The reported incidence of IAH varies widely, ranging from approximately 20% to 65% in critically ill patients, depending on the underlying disease process, surgical pathology, and intensive care setting [4]. Patients undergoing major abdominal surgery, emergency laparotomy, trauma surgery, treatment for severe acute pancreatitis, or massive fluid resuscitation are particularly susceptible [5]. Delayed recognition of elevated IAP has consistently been associated with prolonged intensive care unit stay, increased healthcare utilisation, and poorer clinical outcomes [6]. Measurement of intra-abdominal pressure through the urinary bladder remains the recommended reference standard because of its reliability and reproducibility. However, transvesical pressure measurement requires urinary catheterisation, is intermittent, and remains invasive. Repeated measurements may not always be practical in critically ill patients, particularly in those with urinary tract pathology, pelvic trauma, or contraindications to bladder catheterisation [2,4]. Consequently, considerable interest has focused on identifying reliable, non-invasive bedside methods capable of detecting elevated intra-abdominal pressure at an earlier stage [7]. The physiological relationship between intra-abdominal pressure, intrathoracic pressure, cerebral venous drainage, and intracranial pressure provides the basis for evaluating ophthalmological parameters as potential surrogate markers of elevated IAP. Increased intra-abdominal pressure may impair venous return, resulting in elevated central venous pressure and reduced cerebral venous drainage. This can increase intracranial pressure, leading to expansion of the optic nerve sheath, which is continuous with the intracranial subarachnoid space. Ultrasonographic measurement of optic nerve sheath diameter has therefore emerged as a reliable non-invasive marker of raised intracranial pressure in a variety of neurological and critical care conditions [8]. Similarly, impaired episcleral venous drainage associated with elevated venous pressure may increase intraocular pressure, suggesting that ocular pressure measurements may also reflect systemic haemodynamic changes associated with raised intra-abdominal pressure [9]. Previous investigations have demonstrated increases in both ONSD and IOP during laparoscopic surgery, pneumoperitoneum, and steep Trendelenburg positioning [10]. However, these studies primarily evaluated transient intraoperative increases in intra-abdominal pressure rather than sustained pathological elevations observed in critically ill patients. Evidence evaluating the diagnostic performance of ONSD and IOP as surrogate markers of sustained intra-abdominal hypertension in surgical intensive care unit patients remains limited, particularly in the Indian setting, and few prospective studies have compared the two modalities across different grades of intra-abdominal hypertension. The present prospective diagnostic study was therefore undertaken to evaluate the relationship between intra-abdominal pressure and these two readily available non-invasive bedside parameters, to determine their diagnostic performance for identifying intra-abdominal hypertension, and to assess their potential role as practical bedside screening tools..

Study Design This prospective diagnostic accuracy study evaluated the performance of two non-invasive bedside parameters — intraocular pressure and optic nerve sheath diameter — as surrogate markers for elevated intra-abdominal pressure in surgical intensive care unit patients. It assessed the correlation between the reference standard (IAP) and the index tests (IOP and ONSD) and evaluated their diagnostic accuracy for detecting intra-abdominal hypertension. The study was designed and reported in accordance with the Standards for Reporting Diagnostic Accuracy Studies (STARD 2015) recommendations [12]. Study Setting and Duration The study was carried out in the Departments of General Surgery and Ophthalmology, in collaboration with the Surgical Intensive Care Unit, of a tertiary care teaching hospital in North India, over the study period from 20 January 2023 to 30 April 2024. All recruitment, clinical assessment, and data collection were performed in the ICU using standardized institutional protocols. Study Population and Eligibility Adult patients admitted to the surgical ICU who required intra-abdominal pressure monitoring were categorized as IAH (IAP ≥12 mmHg) or control (IAP <12 mmHg). Inclusion required age ≥18 years, admission to the surgical ICU, an indwelling urinary bladder catheter allowing transvesical IAP measurement, a clinical indication for IAP monitoring, haemodynamic stability permitting ophthalmological assessment, and written informed consent. Patients were excluded for glaucoma or ocular hypertension, previous ocular surgery, significant ocular trauma, orbital pathology, active ocular infection, corneal pathology interfering with tonometry, known intracranial pathology with raised intracranial pressure, severe facial or orbital oedema, pregnancy, or refusal to participate. Sample Size A total of 40 patients were included (20 IAH, 20 controls). Assuming an anticipated correlation coefficient of 0.50 between IAP and ONSD, a 95% confidence level, and 80% power, the minimum required sample size was 29 participants; recruitment was extended to 40 to allow subgroup analysis across IAH grades and to compensate for potential incomplete observations. This calculation was powered for the correlation endpoint; the study was not formally powered for the ROC or sensitivity–specificity analyses, which should therefore be regarded as exploratory. Sampling Technique A balanced two-gate (case–control) sampling strategy was used. Eligible patients were enrolled prospectively into the IAH and control arms until 20 participants had been recruited in each arm, so that both groups were equally represented. Because participants were sampled from above and below the IAH threshold rather than consecutively, the study prevalence is fixed at 50% by design. This deliberate case-selection approach is expected to inflate estimates of sensitivity, specificity, predictive values, and correlation relative to an unselected surgical ICU population, and the diagnostic estimates should be interpreted accordingly. Reference Standard The reference standard was transvesical intra-abdominal pressure measurement using the urinary bladder technique recommended by the WSACS [2]. Measurements were obtained in the supine position, at end expiration, after instillation of 25 mL of sterile saline into the bladder, with the pressure transducer zeroed at the mid-axillary line at the level of the iliac crest, and recorded in mmHg. Index Test 1 — Intraocular Pressure Intraocular pressure was measured using a calibrated handheld Tono-Pen tonometer by an experienced ophthalmologist under standard aseptic precautions, with the patient supine and the head neutral, and the mean value used for analysis. The ophthalmologist was blinded to the measured intra-abdominal pressure and to group allocation. The intraocular pressure measurement was performed within 5 minutes of the paired transvesical intra-abdominal pressure reading, so that the reference standard and the index test reflected the same physiological state. • Index Test 2 — Optic Nerve Sheath Diameter Optic nerve sheath diameter was measured by bedside ocular ultrasonography with a 7.5–13 MHz high-frequency linear probe, 3 mm posterior to the globe in accordance with established techniques [8,11]. Both eyes were examined and the mean binocular ONSD used for analysis. To minimise variability, all examinations were performed by the same ophthalmologist — blinded to the intra-abdominal pressure and to group allocation, thereby eliminating inter-observer variability — with patients supine, the head in neutral alignment, and the head-end elevation kept constant throughout. The ophthalmologist had prior training and clinical experience in ocular ultrasonography ([author to insert years of experience and approximate number of prior scans]), and the optic nerve sheath diameter measurement was performed within 5 minutes of the paired transvesical intra-abdominal pressure reading. Study Variables and Outcomes The primary variable was intra-abdominal pressure; the index variables were mean IOP and mean ONSD. Additional variables were age, sex, body mass index, primary diagnosis, APACHE II score, mechanical ventilation status, emergency surgery, ICU stay, requirement for surgical intervention, and clinical outcome. The primary outcome was the diagnostic accuracy of ONSD and IOP for identifying intra-abdominal hypertension; secondary outcomes were the correlations between IAP, ONSD and IOP, and diagnostic performance across IAH grades Statistical Analysis Data were entered into Microsoft Excel and analysed using IBM SPSS Statistics version 26.0 (IBM Corp., Armonk, NY, USA). Continuous variables were expressed as mean ± standard deviation or median (interquartile range) according to distribution, and categorical variables as frequencies and percentages. Normality was assessed with the Shapiro–Wilk test. Group comparisons used the independent Student's t-test, the Mann–Whitney U test, or the Chi-square/Fisher's exact test, as appropriate. Correlations were assessed using Pearson's correlation coefficient. Trends in IOP and ONSD across ordered grades of intra-abdominal hypertension were tested using the Jonckheere–Terpstra test. Diagnostic accuracy was evaluated with ROC analysis; AUC, sensitivity, specificity, PPV, NPV, and their 95% confidence intervals (Wilson score method for proportions) were calculated, and optimal cut-offs identified using the Youden index. A formal statistical comparison of the two ROC curves (for example, using the DeLong test) was not performed. A two-sided p<0.05 was considered significant. Ethical Considerations The protocol was approved by the Institutional Ethics Committee before recruitment, and the study was prospectively registered with the Clinical Trials Registry–India (CTRI). Written informed consent was obtained from all participants or their legally authorised representatives. The study followed the Declaration of Helsinki (2013 revision), the ICMR Ethical Guidelines, and Good Clinical Practice recommendations. Confidentiality was maintained by unique identification numbers and restricted data access.

Forty adult surgical ICU patients were included (20 IAH, 20 controls); all completed the protocol. The mean age was 48.6 ± 15.2 years, with a male predominance. Primary Surgical Diagnoses [Author to insert Table X: primary surgical diagnoses of enrolled patients — for example perforation peritonitis, severe acute pancreatitis, bowel obstruction, trauma, and postoperative sepsis — with frequencies for the intra-abdominal hypertension group, the control group, and the total cohort.] Baseline Characteristics The groups were comparable for age and sex, but patients with IAH had significantly higher BMI (27.2 ± 3.8 vs. 24.9 ± 3.1 kg/m²; p=0.04) and APACHE II scores (18.6 ± 5.2 vs. 13.2 ± 4.7; p=0.002), a higher proportion of mechanical ventilation (75.0% vs. 45.0%; p=0.05), and longer ICU stays (median 9 vs. 5 days; p=0.01), reflecting greater illness severity (Table 1). Table 1. Baseline demographic and clinical characteristics of study participants (N=40) Variable IAH group (n=20) Control group (n=20) p-value Age (years), mean ± SD 50.8 ± 14.1 46.4 ± 16.2 0.36 Male sex, n (%) 14 (70.0) 13 (65.0) 0.74 BMI (kg/m²), mean ± SD 27.2 ± 3.8 24.9 ± 3.1 0.04 Mechanical ventilation, n (%) 15 (75.0) 9 (45.0) 0.05 Emergency surgery, n (%) 13 (65.0) 8 (40.0) 0.11 APACHE II score, mean ± SD 18.6 ± 5.2 13.2 ± 4.7 0.002 ICU stay (days), median (IQR) 9 (6–14) 5 (3–8) 0.01 Comparison of IAP, IOP and ONSD Mean intra-abdominal pressure was significantly higher in IAH than controls (17.6 ± 4.7 vs. 8.1 ± 2.1 mmHg; p<0.001), as were IOP and ONSD, consistently for both eyes and the mean values (Table 2). Table 2. Comparison of intra-abdominal pressure, intraocular pressure, and optic nerve sheath diameter between study groups Parameter IAH group (n=20) Control group (n=20) p-value Intra-abdominal pressure (mmHg) 17.6 ± 4.7 8.1 ± 2.1 <0.001 Right eye IOP (mmHg) 21.6 ± 4.3 15.4 ± 2.6 <0.001 Left eye IOP (mmHg) 21.2 ± 4.0 15.1 ± 2.5 <0.001 Mean IOP (mmHg) 21.4 ± 4.1 15.3 ± 2.4 <0.001 Right ONSD (mm) 5.68 ± 0.52 4.68 ± 0.39 <0.001 Left ONSD (mm) 5.62 ± 0.49 4.64 ± 0.41 <0.001 Mean ONSD (mm) 5.65 ± 0.48 4.66 ± 0.38 <0.001 Relationship Between IAH Grade and Ophthalmological Parameters Both IOP and ONSD increased progressively with increasing grade of IAH, with the highest values in Grade III–IV (Table 3); the Jonckheere–Terpstra test showed a significant increase in both parameters across ordered grades (p<0.001). Only one patient had Grade IV disease, so the graded relationship in severe disease should be interpreted with caution. Table 3. Distribution of IOP and ONSD according to grade of intra-abdominal hypertension (n=20) IAH grade n Mean IAP (mmHg) Mean IOP (mmHg) Mean ONSD (mm) Grade I (12–15 mmHg) 8 13.6 ± 1.1 18.2 ± 2.3 5.22 ± 0.31 Grade II (16–20 mmHg) 7 17.8 ± 1.3 21.4 ± 2.7 5.73 ± 0.34 Grade III (21–25 mmHg) 4 22.8 ± 1.5 25.8 ± 3.2 6.18 ± 0.29 Grade IV (>25 mmHg) 1 27.0 29.0 6.50 Correlation Between IAP and Non-invasive Parameters On Pearson's correlation, IAP correlated strongly with ONSD (r=0.81, p<0.001) and significantly with IOP (r=0.72, p<0.001); IOP and ONSD were also correlated (r=0.69, p<0.001) (Table 4). Because IAP was sampled at both extremes by design, these coefficients are likely higher than would be observed in an unselected ICU cohort. Table 4. Correlation between intra-abdominal pressure and ophthalmological parameters (N=40) Variable Pearson's r p-value IAP vs Mean IOP 0.72 <0.001 IAP vs Mean ONSD 0.81 <0.001 Mean IOP vs Mean ONSD 0.69 <0.001 Diagnostic Performance of ONSD and IOP On ROC analysis, ONSD provided higher diagnostic performance than IOP (AUC 0.92; 95% CI 0.84–1.00 vs. 0.84; 95% CI 0.72–0.96). At the derived thresholds, ONSD >5.2 mm gave 90.0% sensitivity and 85.0% specificity, and IOP >18 mmHg gave 80.0% sensitivity and 75.0% specificity (Table 5; Figure 1). Table 5. Diagnostic performance of IOP and ONSD for detecting intra-abdominal hypertension Parameter Cut-off Sensitivity, % (95% CI) Specificity, % (95% CI) PPV, % (95% CI) NPV, % (95% CI) LR+ / LR− AUC (95% CI) Mean IOP >18 mmHg 80.0 (58.4–91.9) 75.0 (53.1–88.8) 76.2 (54.9–89.4) 78.9 (56.7–91.5) 3.2 / 0.27 0.84 (0.72–0.96) Mean ONSD >5.2 mm 90.0 (69.9–97.2) 85.0 (64.0–94.8) 85.7 (65.4–95.0) 89.5 (68.6–97.1) 6.0 / 0.12 0.92 (0.84–1.00) The wide 95% confidence intervals reflect the small sample size, so point estimates should be interpreted with caution. The cut-offs were derived and tested within the same cohort and therefore require external validation. Because of balanced 1:1 sampling, the positive and negative predictive values reflect an artificial 50% prevalence and are not generalisable; the corresponding prevalence-independent likelihood ratios were LR+ 3.2 and LR− 0.27 for IOP, and LR+ 6.0 and LR− 0.12 for ONSD. For ONSD, a positive likelihood ratio of 6.0 corresponds to a moderate-to-large increase in the probability of intra-abdominal hypertension when the test is positive, and a negative likelihood ratio of 0.12 to a substantial decrease when negative; the corresponding values for IOP (3.2 and 0.27) indicate weaker rule-in and rule-out ability. However, because the sensitivity and specificity from which they are derived were themselves obtained from a two-gate case–control design, these likelihood ratios carry the same spectrum effect and may overstate performance in an unselected surgical ICU population. The difference in AUC between ONSD and IOP was not formally tested (e.g., by the DeLong method), and their confidence intervals overlap; the description of ONSD as the better-performing parameter should therefore be regarded as exploratory. Summary of Principal Findings Patients with IAH had significantly higher IOP and ONSD than controls; both parameters rose progressively across IAH grades; ONSD showed a stronger correlation with IAP and a higher AUC than IOP; and these findings support ocular ultrasonography as a rapid, non-invasive bedside screening modality in critically ill surgical patients.

This prospective diagnostic accuracy study evaluated intraocular pressure and optic nerve sheath diameter as non-invasive screening markers for elevated intra-abdominal pressure in surgical ICU patients. Both parameters were significantly associated with intra-abdominal pressure, with ONSD showing a stronger correlation and higher diagnostic performance than IOP, and both increased progressively across grades of intra-abdominal hypertension, suggesting a close relationship between rising abdominal pressure and ocular physiological changes. Early recognition of IAH remains a major challenge in critical care. Although transvesical measurement is the recommended reference standard, it is invasive, intermittent, and catheter-dependent [2], driving interest in reliable non-invasive bedside techniques. Patients with IAH had significantly higher IOP and ONSD than controls, consistent with the physiological effects of raised abdominal pressure: increased intrathoracic and central venous pressures impair venous return and cerebral venous drainage, expanding the optic nerve sheath, while impaired episcleral venous drainage raises intraocular pressure [9]. Similar increases occur during laparoscopic pneumoperitoneum and Trendelenburg positioning [10]. Among the two index tests, ONSD showed the stronger correlation and higher AUC, biologically plausible because the optic nerve sheath communicates directly with the intracranial subarachnoid space and responds dynamically to cerebrospinal fluid pressure [8,11], whereas IOP is influenced by additional ocular factors. The correlation coefficients observed here (r = 0.81 for ONSD and r = 0.72 for IOP) are numerically higher than the ONSD–intracranial pressure correlations reported in adult neurocritical care studies, in which coefficients have generally ranged from about 0.45 to 0.66 against invasive intracranial pressure measurement [8,11]. This difference is expected, because the two-gate case–control design used here samples patients from both extremes of the intra-abdominal pressure distribution, which inflates correlation coefficients. The optic nerve sheath diameter cut-off of 5.2 mm derived in the present cohort is slightly lower than most cut-offs reported in the neurocritical care literature for raised intracranial pressure, which typically range from approximately 5.5 to 5.8 mm [8]; a lower threshold is plausible in the present setting because the target condition (intra-abdominal hypertension ≥ 12 mmHg) reflects earlier and more modest transmitted pressure changes rather than established intracranial hypertension. The area under the curve for ONSD (0.92; 95% CI 0.84 to 1.00) is broadly consistent with the diagnostic performance reported for ONSD against direct intracranial pressure measurement in adult neurocritical care cohorts, in which areas under the curve have generally ranged from approximately 0.81 to 0.93 [8,11]. These comparisons should be interpreted with caution because the target condition and the design of the present study differ from those of the neurocritical care literature. The lower sensitivity of IOP is partly explained by the mean IOP in Grade I IAH (18.2 mmHg) lying close to the derived cut-off of >18 mmHg, so that many patients with mild IAH fall near this threshold and are more easily misclassified. The progressive increase across grades is consistent with pressure transmission across interconnected compartments, described as the polycompartment syndrome [6], although few patients were represented in the higher grades. Ocular ultrasonography is widely available, rapid, repeatable, and non-invasive, making it an attractive adjunct in critically ill surgical patients. Where repeated bladder measurements are impractical, bedside ONSD may help identify patients needing closer surveillance or confirmatory IAP measurement, and handheld tonometry may add supportive information. Neither ONSD nor IOP should currently replace transvesical measurement; they are complementary screening tools that may facilitate earlier recognition of IAH. In line with this role, both parameters are best regarded as adjunctive screening markers alongside clinical assessment, rather than as replacements for transvesical measurement. Strengths include the prospective design, standardized protocol, blinded index-test assessment, inclusion of IAH and control groups, simultaneous evaluation of two parameters, and assessment across IAH grades using ROC analysis. Limitations: First, case–control (two-gate) diagnostic accuracy studies, in which participants are recruited separately from above and below the diagnostic threshold, are known to overestimate sensitivity, specificity, predictive values, and correlation compared with consecutive recruitment of an unselected clinical population, and the present balanced 50% design should therefore be regarded as generating exploratory rather than definitive diagnostic estimates. Second, this was a single-centre study with a small sample, limiting generalisability. Third, few patients had severe IAH (particularly Grade IV), reducing precision for advanced disease, and diagnostic performance for abdominal compartment syndrome could not be reliably evaluated. Fourth, the cut-offs were derived and tested within the same cohort, which may overestimate accuracy; external validation is required. Fifth, the two AUCs were not formally compared, so the apparent superiority of ONSD should be interpreted cautiously. Sixth, all ONSD measurements were obtained by a single observer, so inter-observer reproducibility was not assessed. Seventh, the IAH group had higher illness severity, BMI, and mechanical ventilation; positive-pressure ventilation, raised PEEP, hypercapnia, and obesity can independently increase ONSD and IOP and could not be fully adjusted for. Eighth, central corneal thickness, gonioscopy, pachymetry, optic disc evaluation, visual field assessment, and OCT of the retinal nerve fibre layer were not performed. Ninth, patients with glaucoma, previous ocular surgery, and intracranial pathology were excluded, limiting applicability. Finally, as an observational study, causality cannot be inferred.

Both optic nerve sheath diameter and intraocular pressure are significantly associated with intra-abdominal pressure in surgical ICU patients, and both increase progressively with grade of intra-abdominal hypertension. ONSD showed the stronger correlation and higher diagnostic performance and may be the more useful bedside screening marker. Although transvesical measurement remains the reference standard, ocular ultrasonography offers rapid, repeatable, non-invasive, increasingly available bedside assessment and — interpreted alongside clinical assessment — may facilitate earlier recognition of IAH and prompt confirmatory IAP measurement. Given the single-centre design, small sample, and balanced case–control sampling, the proposed thresholds are preliminary and require validation in larger, consecutively sampled multicentre studies before routine use.

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